The present invention generally relates to communication terminals, and more particularly to a communication terminal having the functions of detecting and preventing a line (or line) branch in a telephone or the like.
Recently, various chargeable services utilizing the public line have been provided.
According to such a service, the charge for use is normally made for each line (public line) which is connected to the communication terminal of the subscriber. Since the charge is proportional to the using time and the number of times used, the charge made becomes considerable if the service is used frequently. On the other hand, communications made by unauthorized or illegal use of a line connected to someone else's communication terminal by making a branch have actually occurred, and there are demands to prevent such unauthorized or illegal use of the line.
In the case of a conventional analog telephone line, an authorized subscriber (or user) of the line calls the other end from the communication terminal such as a telephone set and a facsimile via the line and a switching station. The authorized subscriber makes a communication such as a voice communication and a facsimile communication, and the charge for this communication is made to the authorized subscriber of the line. However, information providing services such as the so-called dial Q2 service available in Japan have become available to the user. According to the information providing service, the user calls an information provider of this service, and the information provider provides the user with various information sometimes depending on inputs made by the user from his telephone. According to the information providing service, the charge made is proportional to the amount of information received by the user (caller). Normally, the charge of the information providing service is determined per unit time, and the user, that is, the subscriber of the line, is charged for the information providing service.
When the information providing service such as the above dial Q2 service is used, the charge made to the communication terminal connected to the public line includes in addition to the normal (or base) charge for using the public line the additional charges for using the information providing service. For this reason, the total charge to the subscriber becomes considerably high if the information providing services are frequently used, and unauthorized or illegal use of someone else's line by branching has become a problem. In other words, the unauthorized or unauthorized user steals and uses someone else's line by making an unauthorized branching, so as to reduce his personal costs of receiving the information providing service.
In the case of an integrated services digital network (ISDN) line, an individual user identification (ID) is provided for each connecting unit, so that it is possible to check the user ID in the network (switching station) and preventing the line from being stolen. However, in the case of the analog line, the public switched network cannot check whether or not the communication terminal belongs to the authorized user because there is no user ID, and it is impossible to prevent stealing of the line by an unauthorized or illegal branching made from a communication terminal of an unauthorized user. In addition, if the analog line were to have check functions similar to those provided by the ISDN line, it would become necessary to change the communication procedure (protocol), and this approach would be impractical since it would require considerable modification of the communication terminals and the exchanges.